Many time varying signals are generated with amplitude swings referenced to a D.C. value, with both the D.C. value and signal amplitude excursions conveying information. For example, conventional video signals include a time varying component which conveys image structure information referenced to a D.C. value which establishes the relative brightness of the scene.
In transmitting such signals the D.C. reference value may be lost, thus it becomes necessary at the receiver to reestablish such reference. Typically, in television receivers D.C. restoration is performed by clamping the tips of the horizontal synchronizing pulses to a predetermined D.C. value. This is accomplished by (a) detecting the video signal to generate a pulsed signal coincident with the horizontal synchronizing pulses; (b) applying the received video signal to the input terminal of a coupling capacitor; (c) using the pulsed signal to condition a switch to apply a constant reference value to the output terminal of the capacitor during the horizontal synchronizing intervals; and (d) accessing D.C. restored video signal from the output terminal of the coupling capacitor.
There is a disadvantage to this type of clamping system in that the pulsed signal may introduce switching transients into the video signal. This is particularly true when the clamping circuitry is resident on an integrated circuit including additional analog signal processing circuitry. The potential for introducing such transients is enhanced if the integrated circuitry is realized with high impedance field effect devices.